This is actually a very obscure English–Latin pun.
The Latin for 'now' is nunc; but only if you're speaking (or writing) about the actual present moment: as in "I'm doing it now".
Another sense of the English word 'now' is synonymous with 'any more'. For example: "No one does that now." In other words: people did it in the past, but no one is doing it now.
A third sense is synonymous with 'already': "We are doing that already" means we're doing it now; we've probably been doing it for some time (in the past), and we may continue to do it in the future.
In either of these latter two senses, the Latin word would be iam, as opposed to nunc.
Wikipedia explains this as follows: iam is the Latin word for 'now', but only in the past or future tense. In the present tense, it's nunc. So it's iam if you're talking about yesterday, iam if you're talking about tomorrow, but nunc if you're talking about today.
Note that there is no letter J in Latin, so 'jam' would be spelt iam.
© Haydn Thompson 2017